Which of the following is essential to ensure safe maximum working pressure in a container?

The pressure relief valve is essential for ensuring safe maximum working pressure in a container because it serves as a crucial safety mechanism that prevents overpressure situations. If the pressure within the container exceeds a predetermined safe level, the pressure relief valve automatically opens to allow excess pressure to escape, thereby reducing the risk of container failure or explosion due to overpressurization. This is a fundamental safety feature in many systems that handle gases or liquids under pressure, as it helps to maintain integrity and safety in operations.

While the other components, such as a pressure gauge, excess flow valve, and vapor service valve, are important for different functions within a pressure system, they do not directly manage and prevent overpressure conditions in the way a pressure relief valve does. A pressure gauge measures the pressure within the container, an excess flow valve minimizes the flow in case of a sudden leak, and a vapor service valve controls the release of vapor from the container; however, none of these fulfill the specific safety role that a pressure relief valve does in preventing excessive pressure buildup.